This invention relates to an outer covering or cladding of a building envelope for inclined surfaces such as for a sloped roof. More specifically, this invention relates to a multi-functional cladding passively responsive to sun elevation angles and substantially uniformly ornamental when viewed from common viewing positions. This invention also relates to means and methods to collect, transport, and utilize energy throughout the cladding system. The building envelope includes the walls, roof, windows, and doors of a structure and includes the weather protective surface of the building. The envelope performs many functions including minimizing heat energy transfer between the interior conditioned space and the environment, resisting degradation due to weathering effects as well as presenting an ornamentally appealing surface. Heat energy transfer through the building envelope changes the temperature of the conditioned space. Energy must be expended to maintain the desired temperature of the conditioned space in order to offset energy transfer to or from the environment. Therefore, minimal energy transfer across the envelope is desirable. The rate of energy transfer across the building envelope becomes significant when large temperature differences exist between the ambient environment and the conditioned space. Building environmental systems must be sized according to maximum expected demands in order to ensure comfort levels are maintained. A primary source of heat energy loading on the envelope is due to the absorption of direct solar radiation and secondarily due to the absorption of diffuse solar radiation into the exposed surfaces of the envelope. The outer surface of the envelope is comprised of a cladding system by which tiles, panels, bricks or tiles are arranged over the building substrate in order to provide a contiguous weather resistant layer. Characteristics by which the cladding interacts with incident solar radiation have significant affect on the heat transfer between the interior conditioned space and the outside environment. Minimizing solar heat gain into the conditioned space in summer reduces energy demand on cooling equipment. Maximizing solar heat gain in winter reduces energy demand on heating equipment. Traditionally, the energy required to cool a conditioned space is more expensive than the equivalent energy required for heating the same space due to the type of energy required for each application. Heating is typically accomplished by burning fossil fuel while cooling is typically accomplished using more costly electrical energy. Therefore, envelope performance is designed to minimize solar heat gain in the summer and secondarily to maximize solar heat gain in the winter for much of the globe between about 50-deg North and 50-deg South latitude.
Elevation and azimuth sun angles vary according to time of year, time of day, and the positional latitude on the Earth from which the angles are measured. During summer and especially at summer solstice, the sun is at a higher elevation angle and for longer periods of time during the day than during winter and especially at winter solstice. Peak heating occurs in the hours surrounding solar noon when the elevation angle of the sun is at or near the daily maximum. Energy use to cool a conditioned space in the summer most often reaches a maximum in the early afternoon as a result of the energy absorbed into the active thermal mass of the envelope during the times surrounding solar noon. FIG. 1 illustrates the daily sun path across the sky and throughout the year as measured at 34-deg North latitude and is referred to as a sunpath diagram. The sunpath diagram charts elevation angle (1) and azimuth angle (2) of the sun (3) from winter solstice (5) to summer solstice (4) throughout the day. During summer solstice, the sun elevation angle remains above 40-deg for over seven hours. By comparison, during winter solstice (5) the sun reaches a maximum of only approximately 35-deg elevation angle at solar noon. A cladding system that is responsive to sun elevation angles enables substantial reductions in energy use especially during the cooling season. Both U.S. Pat. No. 3,001,331 granted to Brunton and U.S. Pat. No. 5,511,537 granted to Hively describe cladding methods passively responsive to sun elevation angles each with disadvantages to the present invention disclosed herein.
Energy transfer across the building envelope can be effectively mitigated by the cladding including surface geometry as well as thermal and thermo-optical properties. Some relevant properties according to the present invention are;                a. reflectivity, which relates to the non-wavelength dependent total fraction of incident solar radiation reflected and is measured on a scale of 0 to 1, whereas 1 is a perfect reflector and;        b. transmissibility, which relates to the non-wavelength dependent transmission of solar radiation measured through transparent materials and is measured on a scale of 0 to 1, whereas 1 is perfectly transparent and;        c. absorptivity, which herein relates to the non-wavelength dependent fraction of incident solar radiation not reflected or transmitted and is measured on a scale of 0 to 1, whereas 1 is a perfect absorber and;        d. emissivity, which relates to the non-wavelength dependent effectiveness of emitting or radiating absorbed energy to the surroundings for a given temperature difference between the cladding and surroundings assuming optically thick materials and is measured on a scale of 0 to 1, whereas 1 is a perfect blackbody emitter and;        e. thermal capacitance per unit mass which relates to the temperature rise of the materials for a given unit of energy input and;        f. thermal conductivity, which relates to the time rate of heat energy conducted through materials and into or out of surroundings in physical contact.Since even highly reflective materials absorb some solar radiation, building materials including cladding systems can be advantageously designed to manage the absorbed heat energy. Absorbed heat energy raises the cladding temperature in proportion to the thermal capacity and thermal mass of the cladding. The energy is then typically transferred through conduction and radiation into the building substrate, re-radiated into the surroundings, and or transferred through convection to the air. Roof cladding comprised of a low emissivity surface exposed to the environment will reach a higher peak temperature compared to a similar roof cladding with higher emissivity resulting in increased local air temperature through convective heat transference. The effects of local air heating in regions with a high proportion of absorbing surfaces such as in developed areas is known as the Heat Island Effect and can be a significant source of heat gain into buildings as well as result in decreased air quality.        
Both high reflectivity and high emissivity improve the effectiveness of building cladding to reject solar gain just as high absorptivity and low emissivity increase solar gain. Metals traditionally used for building construction such as cladding include bright zinc galvanized steel (emissivity=0.23 to 0.28), aluminum (emissivity=0.02 to 0.19) and stainless steel (emissivity=0.08 to 0.20). While bare metals are excellent reflectors, these materials do not effectively emit heat energy compared to other building materials such as paint and masonry (emissivity>0.70). Therefore, a cladding system with a bare metal coating exposed to radiant energy will increase in temperature more than would occur for an otherwise equivalent cladding system with a more emissive coating exposed to equivalent radiant energy. Several disadvantages occur as a result of using polished bare metal as a cladding surface such as that described in U.S. Pat. No. 3,001,331 granted to Brunton. The cladding is subjected to larger temperature cycling amplitudes, which degrade the useful life of the system. Local air temperatures increase and air quality decreases as cladding surface temperatures increase. Also, higher cladding temperatures increase the thermal gradient between the inside and outside of the building envelope causing an increase in energy transfer rate. The emissivity of a building cladding becomes more important as the heat capacitance or thermal mass of the cladding is reduced such as that described in U.S. Pat. No. 3,001,331 granted to Brunton. A low thermal mass building cladding increases in temperature greater than that of an otherwise equivalent higher thermal mass building cladding for an equal quantity of energy absorbed. A cladding that both reflects a large fraction of the incident solar energy such as about 0.6 and emits a high fraction of the absorbed solar energy such as about 0.7 back into the environment will be subjected to a lower temperature cycling amplitude compared to the referenced art of equivalent thermal capacity in an identical environment. Limiting the temperature cycling amplitude increases cladding useful life and so is a desirable property of a building cladding. Another disadvantage of the referenced art is that polished metal surfaces must be chosen to withstand the effects of the environment without degrading appreciably in reflectivity, which further limits the applicability in both choice of material and economical manufacturing for a reflective surface. Metals such as copper, iron, steel and aluminum for example do not remain bright when exposed to the weathering effects of the environment unless sufficiently protected with a coating. The layer that develops on the metal surface over time, such as the patina or oxide layer both decreases reflectivity and increases reflectivity. Materials such as bare copper and carbon steel are best suited for substantially vertical surfaces as an ornamental and absorptive surface after a short period of time when exposed to the effects of the weather.
Cladding energy performance increases can be realized by the use of materials exposed to the environment that are greater than about 0.5 reflective and greater than about 0.5 emissive for surfaces designed to reject solar gain. As but some examples of suitable reflective and emissive coatings are light-colored paints, and polymer coatings such as UV stabilized white PVDF (Polyvinylidene Fluoride), epoxy paints pigmented with Titanium Oxides or synthetic pigments of similar reflectivity. The total emissivity of a combination of materials is greatly influenced by the emissivity of the exposed outer surface. Therefore, a sufficiently thick protective coating such as anodize or polymer sheeting applied to an efficient reflector such as a bare metal surface increases both the emissivity as well as the resistance to environmental degradation. Metals with a reflectivity greater than about 0.5 in combination with such a protective coating having greater than about 0.9 transmissibility and greater than 0.5 emissivity functions as a second surface reflector is also a suitable choice. Even mixtures of metal particles in an emissive matrix can be effective reflective and emissive surfaces.
Minimizing energy transfer by thermal conduction between the underlying substrate and the cladding is accomplished by minimizing the surface area of the cladding in contact with the substrate. Minimizing radiation transfer between a reflective and emissive cladding and underlying substrate provides a further method of reducing heat energy transfer across the building envelope. Desirable properties of cladding surfaces exposed to the underlying substrate include high reflectivity such as about 0.5 or greater and low emissivity such as about 0.5 or lower, preferably below 0.3 to further reduce energy transfer between cladding and substrate. Bare metals such as bright zinc galvanized steel and aluminum are well suited for these surfaces.
Most available high reflectivity cladding systems have been incorporated into commercial roof structures, which typically comprise a large area fraction exposed to the sun and have nearly flat roofs that are not commonly visible. These types of roofs are not limited by ornamental requirements and most often are white or lighter in color. Buildings with inclined roofs such as residential structures benefit from the same technologies that have been developed for commercial structures. Many factors are involved when an architect or homeowner chooses a roof cladding for inclined roofs. Since darker roofs are traditionally preferred over lighter colored roofs, methods have been devised to create ornamental, high reflectivity cladding.
U.S. Pat. No. 5,511,537 granted to Hively describes a system in which two aspects negatively affect the functional performance and ornamental appearance as applied to an inclined building surface such as a roof; the overhanging vertical surface and the visible exposure of the reflective surface. An overhanging surface is not effectively self-cleaning except in heavy rains and therefore tend to trap debris. This often leads to degradation and discoloration from environmental fouling and thus results in reduced solar performance and negative ornamental appearance. Further, overhanging surfaces increase manufacturing complexity for molded cladding such as tiles. The exposure of the reflective surface sufficiently enough to be perceived in the visual field of people viewing the cladding from normal viewing positions creates a negative ornamental appearance. The published application WO 2006/1119567 A1 by Totoev discloses a similar cladding unit for a vertical surface such as a wall, which is also comprised of an overhanging surface and therefore manufactured by an extrusion process. The extruding process is capable of producing overhanging surfaces but not capable of producing features required for inclined surface cladding such as the side lap and gutter for tiles.
U.S. Pat. No. 5,303,525 granted to Magee is selectively reflective in response to sun angle. Although an object of the referenced art is to preserve ornamental appeal, the art as described relies on refraction and is therefore very sensitive to environmental fouling. Yet another method for increasing the reflectivity of darker colored cladding utilizes wavelength selective reflectivity in the infrared portion of the spectrum. A representative performance curve for such a system is illustrated as curve (10) of FIG. 2 where the X-axis is wavelength and the right side Y-axis is reflectivity. This technology preserves the visible color ornamental appearance of traditional roofs while being more reflective in the non-visible infrared portion of the spectrum (7). A disadvantage of this technology is that reflectivity is limited to approximately only one-half of the incident solar energy on the building. Curve (8) of FIG. 2 illustrates the normalized energy content on the left side Y-axis versus wavelength of incident solar radiation at ground level. The visible portion of the spectrum is illustrated in FIG. 2 as region (6). A higher performing material might have a performance curve such as that illustrated as curve (9) of FIG. 2 where both visible and infrared portions are effectively reflected such as by a white PVDF surface. Wavelength selective reflectivity addresses the importance of ornamental appeal for visible building cladding but does not exploit the advantages of sun angle responsive selective reflectivity. A preferable cladding rejects the greatest fraction of incident solar radiation during the cooling season while maximizing ornamental appeal.
The sloped roof portion of the building envelope of low-rise buildings represents a large fraction of the surface area exposed to direct solar radiation. Often, additional equipment is located on the roof such as photovoltaic panels and solar thermal panels. Traditionally, this equipment has not been well integrated to the cladding system but rather attached over the cladding system. Ornamental appeal is one consideration for the placement of these systems. As a result, equipment placement is often not ideal from a performance aspect and therefore operates at reduced effectiveness.
U.S. Pat. No. 4,111,188 granted to Murphy discloses modes of collecting solar energy throughout the building cladding such as to preserve ornamental uniformity. An advantage of the referenced art is that the cladding and thermal system are installed above the roof substrate and therefore may be installed easily as a retrofit. Published application US 2006/0288652 A1 by Gurr describes an ornamental electrically heated roof panel for preventing ice dams. A heated cladding and a solar energy collecting cladding can be the same cladding embodiment with the flow of energy established depending on intended use. The utilizing a common cladding system, a multifunctional cladding is possible, which simplifies system design and installation. Further, a cladding with surfaces that remain perceptibly hidden from view can be combined with an energy collecting, distribution, and or dissipating cladding that is multifunctional and that presents a uniformly ornamental appearance when viewed from common viewing positions.